Automatic transmission

ABSTRACT

An automatic transmission includes a first planetary gear set; a second planetary gear set; a third planetary gear set; an input shaft; an output shaft; and five friction elements. A first ring gear is constantly locked. A third sun gear is connected with a first sun gear to define a first rotating member, and a second sun gear is connected with a first carrier to define a second rotating member. The input shaft is constantly connected with the first rotating member, and the output shaft is constantly connected with a third carrier. The five friction elements include a first friction element adapted to selectively connect a second ring gear with a third ring gear, a second friction element adapted to stop a rotation of a second carrier, a third friction element adapted to selectively connect the second carrier with the third ring gear, a fourth friction element adapted to selectively connect the second ring gear with the third carrier, and a fifth friction element adapted to selectively connect the first rotating member with the second carrier. Each of at least seven forward speed-ratios and one reverse speed-ratio is achieved by an engaged state of two friction elements selected from the five friction elements.

BACKGROUND OF THE INVENTION

The present invention relates to a step automatic transmission employedas a transmission for vehicle.

Japanese Patent Application Publication No. 2004-176765 or U.S. Pat. No.6,648,791 (FIGS. 23 to 26) discloses an automatic transmission adaptedto achieve forward seven speed-ratios by using three planetary gearsets. In the automatic transmission disclosed in the above JapanesePatent Application, the forward seven speed-ratios are obtained by usingsix friction elements and three single-pinion-type planetary gear sets.This single-pinion-type planetary gear set has an advantage in transferefficiency and gear noise and also an advantage in durability because ofthe nonnecessity for reducing a diameter of pinion gear. Moreoversimilarly, the automatic transmission disclosed in the above UnitedStates patent achieves forward six speed-ratios to forward eightspeed-ratios by using five friction elements and threesingle-pinion-type planetary gear sets.

SUMMARY OF THE INVENTION

However, in the technique disclosed in the above Japanese PatentApplication, at least six friction elements are necessary to achieve theseven forward speed-ratios. Therefore, there is a problem that thenumber of friction elements is large so that an increase of the numberof components and an increase in axial length are incurred.

Since the number of friction elements provided for achieving the forwardseven speed-ratios is five in the technique of the above United Statespatent, there is an advantage that the number of friction elements issmall as compared with that of the above Japanese Patent Application, sothat the number of components can be reduced. However, in the automatictransmissions shown in FIGS. 23 and 24 of the above United Statespatent, there is a problem that a fuel economy worsens because of a lotof connecting members passing on a radially outer side of the planetarygear sets. For example, regarding FIG. 23, members passing on theradially outer side of a ring gear of center one of three planetary gearsets form a three-layered structure. Generally, in the automatictransmission, lubricating oil is released from a shaft-center side bymeans of centrifugal force and then is collected into an oil panprovided in a lower portion of the automatic transmission via respectiveparts requiring to be lubricated. In the case that the connectingmembers such as a drum member are provided in a multilayer structure onthe radially outer side of planetary gear set, the lubricating oil iseasy to be retained inside the above-mentioned members. Since many ofthese members rotate at the time of vehicle running, there is a problemthat respective frictions are increased to worsen the fuel economy.

On the other hand, in the technique shown in FIG. 25 of the above UnitedStates patent, members passing on the radially outer side of planetarygear set are provided in a two-layered form. Hence, lubricating oil isresistant to the retention (a disrupted flow) as compared to thetechnique of FIG. 23. However, a multi-shaft structure, concretelythree-layered structure at a maximum is formed on a radially inner sideof a sun gear of input-shaft-side one of three planetary gear sets.Hence, dimensions of the sun gear are restricted so that there is aproblem that a degree of freedom to design a gear ratio of theinput-shaft-side planetary gear set is low. In the case of trying tosecure a sufficient value of gear ratio of planetary gear set in thistechnique, another problem is caused that dimensions of the planetarygear set are upsized so as to upsize the outside dimensions of automatictransmission.

It is an object of the present invention to provide an automatictransmission that is capable of achieving seven forward speed-ratios bymeans of three simple planetary gear sets and five friction elements,and that is devised to reduce members passing on the radially outer sideof planetary gear set and/or devised to reduce the number of shaftspassing on the radially inner side of planetary gear set.

According to one aspect of the present invention, there is provided anautomatic transmission comprising: a first planetary gear set includinga first sun gear, a first pinion engaged with the first sun gear, afirst carrier supporting the first pinion, and a first ring gear engagedwith the first pinion and constantly locked; a second planetary gear setincluding a second sun gear connected with the first carrier to define asecond rotating member, a second pinion engaged with the second sungear, a second carrier supporting the second pinion, and a second ringgear engaged with the second pinion; a third planetary gear setincluding a third sun gear connected with the first sun gear to define afirst rotating member, a third pinion engaged with the third sun gear, athird carrier supporting the third pinion, and a third ring gear engagedwith the third pinion; an input shaft constantly connected with thefirst rotating member; an output shaft constantly connected with thethird carrier; and five friction elements including a first frictionelement adapted to selectively connect the second ring gear with thethird ring gear, a second friction element adapted to stop a rotation ofthe second carrier, a third friction element adapted to selectivelyconnect the second carrier with the third ring gear, a fourth frictionelement adapted to selectively connect the second ring gear with thethird carrier, and a fifth friction element adapted to selectivelyconnect the first rotating member with the second carrier, the automatictransmission being adapted to achieve at least seven forwardspeed-ratios and one reverse speed-ratio, each of the at least sevenforward speed-ratios and one reverse speed-ratio being achieved by anengaged state of two friction elements selected from the five frictionelements.

The other objects and features of this invention will become understoodfrom the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a skeleton diagram showing an automatic transmission accordingto a first embodiment of the present invention.

FIG. 2 is a diagram showing a concrete example of an engagement table offriction elements and reduction gear ratios in the automatictransmission according to the first embodiment.

FIG. 3 is a view showing differences between respective first to seventhspeeds according to the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Reference will hereinafter be made to the drawings in order tofacilitate a better understanding of the present invention.

First Embodiment

At first, structures of a shift mechanism of a step automatictransmission according to a first embodiment of the present inventionwill now be explained. FIG. 1 is a skeleton diagram showing the shiftmechanism of the step (multiple-step type) automatic transmissionaccording to the first embodiment. FIG. 2 is a diagram showing aconcrete example of a table regarding engagements of friction elementsand reduction gear ratios (speed reducing ratios) in the automatictransmission according to the first embodiment.

The automatic transmission according to the first embodiment includes afirst planetary gear set PG1, a second planetary gear set PG2 and athird planetary gear set PG3, as a gear train, as shown in FIG. 1. Eachof the three planetary gear sets PG1, PG2 and PG3 is of single-piniontype. First planetary gear set PG1 includes a first sun gear S1, a firstring gear R1, and a first pinion P1 engaged or meshed with first sungear S1 and first ring gear R1. Second planetary gear set PG2 includes asecond sun gear S2, a second ring gear R2, and a second pinion P2engaged with second sun gear S2 and second ring gear R2. Third planetarygear set PG3 includes a third sun gear S3, a third ring gear R3, and athird pinion P3 engaged with third sun gear S3 and third ring gear R3.First, second and third pinions P1 to P3 are supported rotatablyrelative to a first carrier PC1, a second carrier PC2 and a thirdcarrier PC3, respectively. Namely, each carrier PC1, PC2 or PC3pivotally supports the corresponding pinion P1, P2 or P3. Firstplanetary gear set PG1 is disposed on the side of an input shaft IN, andthird planetary gear set PG3 is disposed on the side of an output shaftOUT. Second planetary gear set PG2 is disposed between first planetarygear set PG1 and third planetary gear set PG3.

First sun gear S1 is always connected (engaged) with third sun gear S3(i.e., constantly rotates together with third sun gear S3) to form afirst rotating member M1. First carrier PC1 is always connected withsecond sun gear S2 to form a second rotating member M2. First ring gearR1 is always locked relative to a transmission case 1 (i.e., constantlyfastened to transmission case 1). Input shaft IN is always connectedwith first rotating member M1. Output shaft OUT is always connected withthird carrier PC3.

The automatic transmission further includes one brake, i.e., a secondfriction element B; and four clutches, i.e., first, third, fourth andfifth friction elements A, C, D and E. The first friction element A isprovided between second ring gear R2 and third ring gear R3, and isadapted to selectively connect (engage) second ring gear R2 with thirdring gear R3. The second friction element B is provided between secondcarrier PC2 and transmission case 1, and is adapted to selectively lock(stop) a rotation of second carrier PC2 relative to transmission case 1(i.e., selectively fasten second carrier PC2 to the transmission case).The third friction element C is provided between second carrier PC2 andthird ring gear R3, and is adapted to selectively connect second carrierPC2 with third ring gear R3. The fourth friction element D is providedbetween second ring gear R2 and third carrier PC3, and is adapted toselectively connect second ring gear R2 with third carrier PC3. Thefifth friction element E is provided between first sun gear S1 andsecond carrier PC2, and is adapted to selectively connect first rotatingmember M1 with second carrier PC2.

Output shaft OUT is provided with an output gear or the like to transmitrotational driving force through a differential gear and a drive shaftto a drive wheel, which are not shown. In the case of the firstembodiment, since output shaft OUT is not obstructed by the other memberor the like, the automatic transmission is applicable to both of afront-wheel drive vehicle and a rear-wheel drive vehicle.

The relations in engagements (connections) of the friction elementsunder respective speed-ratios (i.e., respective steps for shift) will beexplained below referring to the engagement table of FIG. 2 (theseengagements for respective speed-ratios are attained by a shift controlsection or means). In the table of FIG. 2, the sign ◯ represents theengagement (engaged state), and the blank represents the disengagement(released state).

At first, the states at the time of forward running will now beexplained. A first-speed (first speed-ratio) is achieved by engagingfirst friction element A and second friction element B. A second-speedis achieved by engaging second friction element B and third frictionelement C. A third-speed is achieved by engaging first friction elementA and third friction element C. A fourth-speed is achieved by engagingthird friction element C and fourth friction element D. A fifth-speed isachieved by engaging third friction element C and fifth friction elementE. A sixth-speed is achieved by engaging first friction element A andfifth friction element E. A seventh-speed is achieved by engaging fourthfriction element D and fifth friction element E. Next, the state at thetime of reverse running is now explained. A reverse-speed is achieved byengaging second friction element B and fourth friction element D.

Next, a concrete example of the reduction gear ratios according to thefirst embodiment will now be explained referring to FIG. 2. Thefollowing explanations are given in the case where a gear ratioρ1=ZS1/ZR1 of first planetary gear set PG1 is equal to 0.50 (i.e.,ρ1=ZS1/ZR1=0.50), a gear ratio ρ2=ZS2/ZR2 of second planetary gear setPG2 is equal to 0.65 (i.e., ρ2=ZS2/ZR2=0.65), and a gear ratioρ3=ZS3/ZR3 of third planetary gear set PG3 is equal to 0.55 (i.e.,ρ3=ZS3/ZR3=0.55). Where, each of ZS1, ZS2, ZS3, ZR1, ZR2 and ZR3represents the number of teeth of the corresponding gear.

A reduction gear ratio i1 of the first-speed in the forward running isexpressed by a formula: i1=(1+ρ1) (1+ρ3)/(ρ3+ρ1ρ3−ρ1ρ2). By assigningthe concrete numerical values to this formula, reduction gear ratio i1of the forward first-speed is calculated as i1=4.650. The inverse ofreduction gear ratio i1 is equal to 0.215.

A reduction gear ratio i2 of the second-speed in the forward running isexpressed by a formula: i2=(1+ρ3)/ρ3. By assigning the concretenumerical values to this formula, reduction gear ratio i2 of the forwardsecond-speed is calculated as i2=2.818. The inverse of reduction gearratio i2 is equal to 0.355.

A reduction gear ratio i3 of the third-speed in the forward running isexpressed by a formula: i3=(1+ρ1) (1+ρ3)/(ρ1 (1+Σ3)+Σ3). By assigningthe concrete numerical values to this formula, reduction gear ratio i3of the forward third-speed is calculated as i3=1.755. The inverse ofreduction gear ratio i3 is equal to 0.570.

A reduction gear ratio i4 of the fourth-speed in the forward running isexpressed by a formula: i4=(1+ρ1) (ρ2+ρ3+ρ2ρ3)/(ρ3 (1+ρ1) (1+ρ2)+ρ1ρ2).By assigning the concrete numerical values to this formula, reductiongear ratio i4 of the forward fourth-speed is calculated as i4=1.385. Theinverse of reduction gear ratio i4 is equal to 0.722.

A reduction gear ratio i5 of the fifth-speed in the forward running isexpressed by a formula: i5=1.0. Without assigning the concrete numericalvalues to this formula, reduction gear ratio i5 of the forwardfifth-speed is equal to 1.000. The inverse of reduction gear ratio i5 isequal to 1.000.

A reduction gear ratio i6 of the sixth-speed in the forward running isexpressed by a formula: i6=(1+ρ1) (1+ρ3)/((1+ρ1) (1+ρ3)+ρ2). Byassigning the concrete numerical values to this formula, reduction gearratio i6 of the forward sixth-speed is calculated as i6=0.782. Theinverse of reduction gear ratio i6 is equal to 1.279.

A reduction gear ratio i7 of the seventh-speed in the forward running isexpressed by a formula: i7=(1+ρ1)/(1+ρ1+ρ2). By assigning the concretenumerical values to this formula, reduction gear ratio i7 of the forwardseventh-speed is calculated as i7=0.698. The inverse of reduction gearratio i7 is equal to 1.433.

A reduction gear ratio iR of the reverse-speed is expressed by aformula: iR=−(1+ρ3)/(ρ2ρ3). By assigning the concrete numerical valuesto this formula, reduction gear ratio iR of the reverse-speed iscalculated as iR=−4.615. The inverse of reduction gear ratio iR is equalto −0.217.

Next, a characteristic of the respective reduction gear ratios in thefirst embodiment is explained referring to FIG. 3. FIG. 3 is a tableshowing interrelations among the respective speed-ratios obtained by theautomatic transmission in the first embodiment. It is proper that aratio between speed-ratios (i.e., relation among respectivespeed-ratios, also called a step ratio) is evaluated based on how closea so-called V1000 is to arithmetic series (arithmetic progression). ThisV1000 is comparable to a vehicle-speed range which each speed-ratio isin charge of (a vehicle-speed range which each step for shift covers).That is, in the case where the V1000 is allocated in a manner ofarithmetic series, each of the in-charge vehicle-speed ranges inrespective speed-ratios has an equivalent width to one another. In thiscase, particularly; upshifts become rhythmical, and a selection ofspeed-ratio on a downhill or uphill road is conducted with nodifficulty, so that the automatic transmission having a superiordrivability can be achieved.

In order to quantitatively determine how the V1000 has deviated ordeparted from the arithmetic series, the following procedure is used.That is, the inverses of reduction gear ratio values under therespective speed-ratios are normalized by regarding the inverse ofreduction gear ratio value under the highest-step speed-ratio (i.e.,seventh speed) as a value equal to 1, and then a deviation degree(amount) of these normalized values from the arithmetic series isquantitatively calculated. In the automatic transmission according tothe first embodiment, a standard deviation calculated is equal to 0.041which is recognized as an extremely small value.

Effects According to the First Embodiment

{circle around (1)} Effects by Virtue of Structural Skeleton as a Whole

In the first embodiment, the automatic transmission capable of attainingthe seven speeds of forward running and the one speed of reverse runningcan be realized with proper reduction gear ratio values being ensured;although the automatic transmission is constructed by a limited numberof simple constructional elements, namely, the three sets of simpleplanet gears (three single-pinion-type planetary gear sets) and the fivefriction elements.

{circle around (2)} Effects by Virtue of the Usage of Three SimplePlanetary Gear Sets

Because of the usage of the three sets of simple planet gears (the usageof three single-pinion-type planetary gear sets), a gear noise and atransfer efficiency can be improved as compared to the case where doublepinions (double-pinion-type planetary gear set) are used. Further,because a diameter of the pinion does not need to be reduced in thisembodiment, a gear durability can be enhanced.

{circle around (3)} Effects Based on Ratio Coverage in the ForwardRunning

A ratio coverage (gear-ratio width) of the forward running is defined bydividing the reduction gear ratio of the lowest-speed (step) by thereduction gear ratio of the highest-speed (step), i.e., the reductiongear ratio of the lowest-speed/the reduction gear ratio of thehighest-speed. It can be described that a compatibility between anaccelerating performance at the time of vehicle start and a fuel economyat the time of high speed cruise of vehicle becomes better, and also adegree of freedom to set the gear ratio values in respective forwardspeed-ratios becomes higher; as the value of ratio coverage becomesgreater. As concrete numerical values in the first embodiment, thereduction gear ratio of the forward first-speed is equal to 4.650 andthe reduction gear ratio of the forward seventh-speed is equal to 0.698.In this embodiment, the ratio coverage from first-speed to seventh-speedis equal to 6.66, and hence a sufficient ratio coverage can be ensured.Therefore, for example, the automatic transmission according to thefirst embodiment is useful also as a transmission for a vehicle equippedwith a diesel engine as its power source, although a width of rotationalspeed (number of revolutions) of diesel engine is narrower than that ofgasoline engine and a torque of diesel engine is higher than that of agasoline engine having the same engine displacement.

Moreover, in the case where the gear ratio value of the low-speed sideis great relative to the ratio coverage, a torque transmitted to a finalgear becomes relatively great. Hence, this case requires a sufficientstrength of the automatic transmission or propeller shaft, so that thewhole of vehicle body is upsized. That is, it is preferable that thegear ratio value (value of speed ratio) of the lowest-speed is not sogreat under the same condition of ratio coverage. In an automatictransmission shown by FIG. 23 in the document of U.S. Pat. No.6,648,791, a gear ratio of the highest-speed (highest step for shift) isequal to 1. Hence in this technique, when trying to enlarge the ratiocoverage, a gear ratio (value) of the lowest-speed (lowest step forshift) needs to be enlarged, so that the upsizing of the automatictransmission and the propeller shaft is caused. On the other hand, inthe automatic transmission according to the first embodiment of thepresent invention, a sufficient ratio coverage can be ensured withoutthe necessity of enlarging the gear ratio of the lowest-speed so much.

{circle around (4)} Effects Based on 1-R Ratio

A value of 1-R ratio (Reverse-speed/First-speed) is a value near 1,concretely, equal to 0.99. Accordingly, an acceleration feel of vehiclerelative to a depressing adjustment of accelerator pedal does notgreatly vary between at the time of forward running and at the time ofreverse running. Therefore, the problem that the drivability worsens canbe avoided.

{circle around (5)} Effects Based on Standard Deviation Related to theV1000

Since the standard deviation can be made an extremely small value (forexample, 0.041 in this embodiment), the widths of the vehicle-speedranges in the respective speed-ratios (the respective steps for shift)are equivalent to one another. Accordingly, particularly, the upshiftattains a rhythmical shift, and the selection of reduction gear ratio isconducted with no difficulty on a downhill or uphill road, so that theautomatic transmission having a superior drivability can be provided.

{circle around (6)} Effects Based on the Number of Changeovers Among theFriction Elements at the Time of Shift

If one or more friction element is released and two or more frictionelements are engaged at the time of shift, or if two or more frictionelements are released and one or more friction element is engaged at thetime of shift; a torque control and a control for the engaging andreleasing timings of friction elements become complicated. Hence, from aviewpoint of avoidance of the complication of shift control, it isfavorable that one friction element is released and another frictionelement is engaged at the time of shift. That is, it is favorable that aso-called double-changeover is avoided. In the first embodiment, theshifts between the forward first-speed and the forward second-speed areperformed under the condition where second friction element B ismaintained in engaged state. The shifts among the forward second-speedto the forward fifth-speed are performed under the condition where thirdfriction element C is maintained in engaged state. Moreover, the shiftsamong the forward fifth-speed to the forward seventh-speed are performedunder the condition where fifth friction element E is maintained inengaged state. Namely, each shift between adjacent two speed-ratios(gear steps) among the forward first-speed to seventh-speed can beachieved by releasing one friction element and by engaging one frictionelement. Accordingly, each of all the shifts between adjacent twospeed-ratios of forward running is performed by means of only thechangeover from one friction element to the other one friction element.Therefore, the control during the shift can be prevented from beingcomplicated.

{circle around (7)} Effects Based on Layout

(i) In the automatic transmission according to the first embodiment, ona radially outer side of the three planetary gear sets, connectingmembers are disposed so as not to become in a three-layered form, asshown in the skeleton diagram of FIG. 1. That is, the number of theconnecting members radially covered or overlapped with each otherradially outside each planetary gear set is smaller than three.Accordingly, it becomes difficult to cause the retention of alubricating oil (disrupted flow of lubricating oil), so that the fueleconomy can be improved by reducing the frictions.

(ii) Moreover, the rotating member passing on the outer peripheral sideof the planetary gear sets is formed in a single-layered structure, asshown by the skeleton diagram of FIG. 1. Generally in the automatictransmission, lubricating oil is always supplied to respective rotatingelements such as gears and bearings (not shown) for the purpose ofcooling, lubrication and the like. This lubricating oil is generallysupplied from a shaft-center side of the transmission by means ofcentrifugal force. At this time, if an efficiency of discharge(retrieving performance) of lubricating oil becomes worsened on theouter peripheral side of the planetary gear sets, oil temperature risesso that a durability of friction elements, bearings and the like isreduced. The rotating member passing on the outer peripheral side of theplanetary gear sets forms the single-layered structure in the firstembodiment as mentioned above, and particularly, no rotating member isdisposed at the outer peripheral of first planetary gear set PG1.Accordingly, the discharging efficiency of lubricating oil is notworsened so that the temperature rise is suppressed to improve thedurability.

(iii) Members passing on a radially inner side of the three planetarygear sets are provided in a double-shaft structure at a maximum. Thatis, the number of members (shafts) passing through a radially insidespace (of the sun gear) of each planetary gear set is smaller than orequal to 2. Accordingly, dimensions (size) of each sun gear are notrestricted as compared with the technique disclosed in the document ofU.S. Pat. No. 6,648,791, and a degree of freedom to design the ratiobetween teeth numbers in each planetary gear set is high so that adegree of freedom to design the automatic transmission can be enhanced.

(iv) The automatic transmission according to the first embodiment can bedesigned to allow torque to be inputted to one side of the planetarygear sets and then to be outputted from another side of the planetarygear sets. Accordingly, the automatic transmission according to thefirst embodiment is applicable to both of a front-wheel drive vehicleand a rear-wheel drive vehicle, namely can be widely applied.

{circle around (7)} Effects From a Viewpoint of the Number of FrictionElements

The number of friction elements in the first embodiment is five, andthese friction elements include one brake as second friction element B.Since the brake is provided as one of the five friction elements; theincrease of the number of seals for rotation and the increase ofcentrifugal canceling mechanisms can be suppressed as compared with thecase where the number of clutches is great. Thereby, the increase of thenumber of components and the increase in axial length can be suppressedwhile enhancing the fuel economy.

This application is based on a prior Japanese Patent Application No.2007-309537 filed on Nov. 29, 2007. The entire contents of this JapanesePatent Application are hereby incorporated by reference.

Although the invention has been described above with reference tocertain embodiments of the invention, the invention is not limited tothe embodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art inlight of the above teachings. The scope of the invention is defined withreference to the following claims.

1. An automatic transmission comprising: a first planetary gear set including a first sun gear, a first pinion engaged with the first sun gear, a first carrier supporting the first pinion, and a first ring gear engaged with the first pinion and constantly locked; a second planetary gear set including a second sun gear connected with the first carrier to define a second rotating member, a second pinion engaged with the second sun gear, a second carrier supporting the second pinion, and a second ring gear engaged with the second pinion; a third planetary gear set including a third sun gear connected with the first sun gear to define a first rotating member, a third pinion engaged with the third sun gear, a third carrier supporting the third pinion, and a third ring gear engaged with the third pinion; an input shaft constantly connected with the first rotating member; an output shaft constantly connected with the third carrier; and five friction elements including a first friction element adapted to selectively connect the second ring gear with the third ring gear, a second friction element adapted to stop a rotation of the second carrier, a third friction element adapted to selectively connect the second carrier with the third ring gear, a fourth friction element adapted to selectively connect the second ring gear with the third carrier, and a fifth friction element adapted to selectively connect the first rotating member with the second carrier, the automatic transmission being adapted to achieve at least seven forward speed-ratios and one reverse speed-ratio, each of the at least seven forward speed-ratios and one reverse speed-ratio being achieved by an engaged state of two friction elements selected from the five friction elements.
 2. The automatic transmission as claimed in claim 1, wherein the seven forward speed-ratios are achieved by concurrent engagements of the first friction element and the second friction element, concurrent engagements of the second friction element and the third friction element, concurrent engagements of the first friction element and the third friction element, concurrent engagements of the third friction element and the fourth friction element, concurrent engagements of the third friction element and the fifth friction element, concurrent engagements of the first friction element and the fifth friction element, and concurrent engagements of the fourth friction element and the fifth friction element.
 3. The automatic transmission as claimed in claim 2, wherein the one reverse speed-ratio is achieved by concurrent engagements of the second friction element and the fourth friction element.
 4. The automatic transmission as claimed in claim 1, wherein each of all the shifts between adjacent two speed-ratios among the at least seven forward speed-ratios is performed by releasing one friction element of the at least five friction elements and by engaging the other one friction element of the at least five friction elements. 